1. Field of the Invention
The invention generally relates to a method for forming a diode from a portion of the gate layer of a semiconductor device, and to the diode formed within that portion of the gate layer.
2. Description of the Related Art
A diode is frequently employed as part of an integrated circuit, and may serve a variety of different functions including, but not limited to measurement of temperature of the nearby surrounding area on a chip on which it is fabricated, rectification of alternating current to direct current, band gap diode applications, and generator circuit diode applications.
A conventional diode formed on a silicon substrate typically contains a P+ shallow junction region, an N-well, and an N+ shallow junction region. A well is a defined region infused with a dopant. A dopant is a substance introduced into the silicon that is typically ionic in nature. FIG. 1 depicts a conventional diode 100 formed within a silicon substrate 110. An N-well 105 is doped with N ions. The diode contains a P+ shallow junction region 120, a terminal 140 to connect the P side of the diode externally, the N-well 105, an N+ shallow junction region 130 and a terminal 150 to connect the N side of the diode externally. The P+ shallow junction region 120 is heavily doped with P ions, usually as a result of implantation of an acceptor dopant such as boron. In FIG. 1, the P+ shallow junction region 120 contains a first sub-region 122 that typically has substantially uniform doping density, and a second sub-region 124 of progressively decreasing doping density one gets closer to boundary 170 between P-doped region and N-well. A region wherein doping density changes with distance traversed, is said to have a “doping profile.” The portion of a diode doped with P ions is usually called the “anode” of the diode. The N+ shallow junction region 130 is heavily doped with N ions, usually as a result of implantation of a donor dopant such as arsenic or phosphorus. The density of N ions in the N+ shallow junction region 130 is typically higher than the density of N ions in the N-well 105. The portion of a diode doped with N ions is usually called the “cathode” of the diode. In FIG. 1 the cathode includes the N-well 105 and the N+ shallow junction region 130. Shallow Trench Isolation structures (STI) 160, typically consisting of an oxide of the substrate, serve to electrically insulate the regions 120 and 130 respectively, from regions of opposite polarity doping, where such insulation is desired.
Doping enhances the number of electrical carriers of one type—either positive-type or negative-type donor. N-doped regions typically have an excess of free electrons (negatively charged electrical carriers), while P-doped regions typically have an excess of “holes” (positively charged electrical carriers). When a P+ shallow junction region and an N-well are placed adjacent to one another, some of the free electrons from the N-well combine with holes from the P+ shallow junction region, forming a “depletion zone”, which region is of essentially net zero charge. FIG. 1 shows a depletion zone 180.
A conventional diode allows current flow in one direction. (Other types of diodes, such as a Zener diode and a current regulator diode, may allow current to pass in both directions.) A reverse bias may be applied to the diode, wherein the electrical potential at the terminal 150 is higher than the electrical potential at the P+ shallow junction region terminal 140. Holes (positive carriers) within the P+ shallow junction region face an “uphill potential difference” when crossing boundary 170 from the P+ shallow junction region 120 to the N-well 105, and so will not travel from the P+ shallow junction region 120 to the N-well 105. Similarly, free electrons (negative carriers) in the N-well face an “uphill potential difference” when crossing boundary 170 from the N-well 105 to the P+ shallow junction region 120. Therefore, free electrons will not travel from the N-well 105 to the P+ shallow junction region 120. Hence, when reverse bias is applied via the contacts 140 and 150, there will be essentially zero current flow.
Applying a forward bias via the contacts 140 and 150 means that the electrical potential is higher on the P+ shallow junction side of the diode than on the N-well side of the diode. Forward bias causes a current to flow through the diode, because holes from the P+ shallow junction region 120 face a “downhill” potential difference when crossing the boundary 170 from the P+ shallow junction region 120 to the N-well 105. Free electrons face a “downhill” potential difference when crossing the boundary 170 from the N-well 105 to the P+ shallow junction region 120. Hence there is a net current flow across the boundary 170. Thus, the diode may serve as a current switch, allowing current to flow across the boundary 170 and through the contacts 140, 150 when forward bias is applied, and preventing current from flowing when reverse bias is applied.
In forward bias, the amount of current that flows depends on the voltage applied across the diode terminals. Above a “threshold voltage” (typically approximately 0.5 volts), the current flowing increases exponentially as the voltage across the terminals is increased. At voltages slightly above the threshold voltage (approximately 0.8-0.9 v), the diode behaves as a conductor, showing only a very small resistance to the movement of holes and free electrons across the boundary 170.
One function of that a diode may serve in an integrated circuit is a temperature sensor. To function as a temperature sensor, the diode's characteristic electrical behavior in response to temperature must be determined. To this end, a diode may be connected to a constant current circuit, which circuit may supply a forward bias voltage to the diode to maintain a constant current flow. FIG. 2 depicts an arrangement wherein current is supplied to a diode 210 by a constant current supply 220. A voltage-measuring device 230 measures forward bias supplied by the constant current supply 220. Within the approximate forward bias voltage range of 0.5-0.8V, the bias voltage needed to maintain a predetermined current is well-known to be inversely proportional to diode temperature, with the voltage needed being reduced by approximately 0.002V for each 1° C. rise in diode temperature. By varying the temperature of the diode and measuring the voltage needed to maintain a predetermined current flow, a temperature-voltage characteristic (also called a temperature-voltage characteristic curve) may be established. Once the temperature-voltage characteristic has been determined, the diode may then be used as a temperature measurement device for measuring, e.g., the temperature of the silicon chip on which the diode resides.
In a particular application as a temperature measurement device in an integrated circuit device, the measured temperature may, in turn, be used to control other devices whose operation depends on temperature. For example, the temperature measured by a diode temperature sensor may be used to control, e.g., the self-refresh current in a dynamic random access memory (DRAM), DRAM refresh period, or the current utilized by a logic circuit such as a central processing unit (CPU).
As device geometries continue to shrink with advances in circuit design and manufacturing techniques, the operating characteristics of diodes are affected. For example, a reduction in junction depth causes the doping profile to change within a conventional diode, which may affect the operating characteristics of the diode. A shallow junction depth has been found to result in a diode whose characteristics, such as voltage v. temperature, are not consistent among chips of identical design and layout, but instead vary considerably from chip to chip. With manufacturing geometries of integrated circuits becoming smaller (e.g., line widths less than 70 nm), design constraints typically dictate corresponding diode junction depths that are shallow (e.g., junction depths smaller than 500 A). In order to use such diodes for measurement purposes, inconsistencies in diode characteristics would necessitate calibration of each individual diode, which is impractical when manufacturing large quantities of an integrated circuit. Hence shallow junction depth becomes problematic for diodes to be used in measurement applications on an integrated circuit. Furthermore, variations in the shallow trench isolation structure may be a contributing cause of non-linear doping profiles in a shallow junction situated adjacent to a shallow trench isolation structure, which in turn may result in inconsistent diode characteristics among diodes within an integrated circuit, or inconsistent diode characteristics between diodes of different integrated circuits.
Higher dopant density and deep junction tend to improve consistency of the diode temperature-voltage characteristic among integrated circuits of identical design and layout. However, as layout geometries become smaller, the dopant density of shallow junctions is correspondingly limited by design rules. Hence, the dopant density cannot simply be increased to offset inconsistencies in diode characteristics, such as the temperature-voltage characteristic, resulting from shallow diode junction depths. Therefore there is a need for a diode whose characteristics, such as voltage versus temperature, are consistent.